docs: def nm_width(center, width, units="wn") -> float:
      """Given a center and width, in energy units, get back a width in nm.
  
      Parameters
      ----------
      center : number
          Center (in energy units).
      width : number
          Width (in energy units).
      units : string (optional)
          Input units. Default is wn.
  
      Returns
      -------
      number
          Width in nm.
      """
      red = wt_units.converter(center - width / 2., units, "nm")
      blue = wt_units.converter(center + width / 2., units, "nm")
      return red - blue